Hostile attacks against a computer program are prevented when the program is executed in a computing environment that is controlled by the attacker. A preposition is run in a secure computing environment instead of the original call site, i.e. the IF antecedent, which has a TRUE consequence and a FALSE consequence are run in a secure computing environment. Embodiments of the invention also allow an insecure call site to detect modifications by a hostile attacker surreptitiously. In embodiments of the invention, a script is generated by a script generator from the IF antecedent and TRUE and FALSE consequence source code, for example relative to a call site. The original source code is modified to call the script engine, rather than the preposition. At run-time a script engine executes this script.